1. Field of the Invention
The present invention relates to a liquid electrophotographic printer, and more particularly, to a liquid electrophotographic printer having a development system that includes three rollers.
2. Description of the Related Art
Electrophotographic printers such as laser printers output a desired image by forming a latent electrostatic image on a photoreceptor medium such as a photoreceptor drum or photoreceptor web, developing the latent electrostatic image with a predetermined color toner, and transferring the toner image to a print paper. Electrophotographic printers are classified into a dry type or liquid type according to the toner used. The liquid type printer uses an ink containing a volatile liquid carrier and toner particles in a predetermined ratio to implement a color image with excellent print quality. The dry type printer uses toner in a powder form.
FIG. 1 shows a conventional liquid electrophotographic printer, which uses a photoreceptor web 10 as a photoreceptor medium. The photoreceptor web 10 circulates around a continuous path by being supported by three rollers 11, 12 and 13, and a main charger 20 is provided adjacent to the photoreceptor web 10 to uniformly charge the photoreceptor web 10 to a predetermined potential. Laser scanning units (LSUs) 30a, 30b, 30c and 30d for emitting light beams onto the charged photoreceptor web 10 to form a latent electrostatic image, and development units 40a, 40b, 40c and 40d for developing the latent electrostatic image as a toner image with a predetermined color ink are provided below the photoreceptor web 10. The conventional liquid electrophotographic printer includes a drying unit 50 for drying the developed image, a transfer unit 60 for printing the dried image on a print paper P, and an eraser 70 for removing the remaining latent electrostatic image from the surface of the photoreceptor web 10. For a color printer, the four development units 40a, 40b, 40c, and 40d for sequentially developing four color toner images of yellow (Y), cyan (C), magenta (M), and black (K), respectively, to implement a multi-color image are provided. The four LSUs 30a, 30b, 30c, and 30d are provided corresponding to the number of the development units.
The drying unit 50 includes a drying roller 51 which rotates in contact with the photoreceptor web 10 and absorbs the liquid carrier from the surface of the photoreceptor web 10, and a heat roller 52 for evaporating the liquid carrier absorbed by the surface of the drying roller 51 by heating.
The transfer unit 60 includes a transfer roller 61 which rotates in contact with the photoreceptor web 10 and transfers the toner image formed on the surface of the photoreceptor web 10 to the print paper P, and a fusing roller 63 for hot pressing the print paper against the transfer roller 61. Reference numerals 62 and 64 are cleaning rollers for cleaning the transfer roller 61 and the fusing roller 63, respectively.
The four development units 40a, 40b, 40c, and 40d are arranged below the photoreceptor web 10 in series in a circulation direction of the photoreceptor web 10. In a lower portion of the development units 40a, 40b, 40c and 40d, ink reservoirs 80a, 80b, 80c and 80d which contain Y, C, M, and K inks, are provided, respectively. In the inks contained in the ink reservoirs 80a, 80b, 80c and 80d, toner particles are mixed with a pure liquid carrier in a concentration amount of 2.5-3% solution by weight.
The structure of the development units 40a, 40b, 40c, and 40d will be described with reference to the development unit 40a for developing a yellow (Y) toner image, referred to herein as a Y-development unit. Referring to FIG. 2, a developer roller 41, a squeeze roller 43 and a topping corona 45 are installed in the upper portion of the Y-development unit 40a. An ink supply nozzle 49 for supplying an ink to the gap between the photoreceptor web 10 and the developer roller 41 is installed adjacent to the developer roller 41. A cleaning roller 47 is installed underneath the developer roller 41. A cleaning blade 48 is affixed to the lower portion of the squeeze roller 43. The developer roller 41 serves to make the ink adhere to a latent electrostatic image region of the photoreceptor web 10. The squeeze roller 43 squeezes the liquid carrier out of the ink adhering to the photoreceptor web 10. The topping corona 45 recharges the photoreceptor web 10 to a predetermined potential for development of another color image. The cleaning roller 47 and blade 48 are used for removing the excessive ink or liquid carrier remaining on the surface of the developer roller 41 and the squeeze roller 43, respectively.
A development system of the conventional liquid electrophotographic printer having the configuration described above will now be described in greater detail.
The photoreceptor web 10 is charged to a potential of about 650 volts by the main charger 20. The Y-LSU 30a emits a beam onto the charged surface of the photoreceptor web 10 to form a latent electrostatic image of Y color. The Y-LSU 30a selectively erases the surface potential of the photoreceptor web 10 to form a latent electrostatic image, so that the potential of an image region in which a latent electrostatic image is formed drops to about 100 volts or less.
The latent electrostatic image is developed into a Y-image by the Y-development unit 40a. In particular, the surface of the developer roller 41 is charged to a potential VD of about 500 volts, and the developer roller 41 rotates in a circulation direction of the photoreceptor web 10 with a development gap G of 100-200 xcexcm from the photoreceptor web 10. When a Y-ink is supplied into the gap between the photoreceptor web 10 and the developer roller 41 by the ink supply nozzle 49, a nip N having about 6-mm width is formed between the photoreceptor web 10 and the developer roller 41. The toner particles contained in the ink are generally charged to a positive potential. Thus, toner particles selectively adhere to an image region B having a potential relatively lower than that in a non-image region A in which no latent electrostatic image is formed, so that a high-concentration toner image is developed.
During this development process, excess ink adhering to the surface of the rotating developer roller 41 is removed by the cleaning roller 47. The squeeze roller 43 squeezes the liquid carrier out of the developed toner image region by compression, so that a toner image having a concentration of about 50% is formed in the image region B of the photoreceptor web 10 passed through the squeeze roller 43. The liquid carrier squeezed by the squeeze roller 43 is also removed from the surface of the squeeze roller 43 by the cleaning blade 48. The ink and liquid carrier removed by the cleaning roller 47 and blade 48 is recovered into the ink reservoir 80a. 
After the Y-image is developed, the photoreceptor web 10 is charged again to a predetermined potential by the topping corona 45 for development of a next color image, i.e., a C-image. The C-LSU 30b emits a light beam onto the surface of the photoreceptor web 10 to form a latent electrostatic image of C color. The latent electrostatic image is developed into a C-toner image by the C-development unit 40b. 
As described above, the images of four colors are sequentially developed in the order of Y, C, M, and K, so that a full color image is formed. The developed color image is dried in the drying unit 50 to the extent of appropriately performing a subsequent transfer process, and in turn transferred to the print paper P in the transfer unit 60.
However, the conventional liquid electrophotographic printer which operates with the configuration, as described above, has the following problems.
First, two layers are formed on the surface of the photoreceptor web 10 passed through the developer roller 41, including a high-concentration ink layer adhering to the image region B, and a liquid carrier layer covering the non-image region A and the ink layer. Here, no toner particles should exist in the liquid carrier layer. However, it is difficult to completely remove toner particles from the liquid carrier layer, and thus actually about 0.5% toner particles exist in the liquid carrier. Accordingly, even after the liquid carrier is mostly removed by the squeeze roller 43, a thin liquid carrier film containing toner particles remains in the non-image region A of the photoreceptor web 10. As the photoreceptor web 10 circulates, the toner particles in the thin liquid carrier film are carried into the C-development unit 40b and are mixed with toner particles of another color. As a result, the C-development unit 40b, M-development unit 40c, and K-development unit 40d arranged in the order, and the inks contained in the development units are sequentially contaminated. In addition, toner particles remaining in the non-image region A are also transferred to the print paper P in the transfer unit 60, so that the non-image region of the print paper P is smeared.
Second, when the liquid carrier is squeezed out of the image region B of the photoreceptor web 10 by the squeeze roller 43, a part of the image may adhere to the surface of the squeeze roller 43 by compression force applied to the image region B of the photoreceptor web 10. In this case, the part of the image remaining on the surface of the squeeze roller 43 may be transferred onto a next color image.
Third, when the liquid carrier is squeezed out of the image region B of the photoreceptor web 10 by the squeeze roller 43, the image formed in the image region B is compressed and thus forced beyond its intended edge, so that it extends into the neighboring non-image region or other color image regions.
The problems described above degrade the overall quality of color images.
To solve the problems of the prior art, it is an aspect of the present invention to provide a liquid electrophotographic printer adopting a development system including three rollers, one of which is a toner removal roller, in which contamination of a development unit is prevented and image quality improved.
To achieve the foregoing aspect of the present invention, there is provided a liquid electrophotographic printer comprising: a photoreceptor web circulating around a continuous path, having a non-image region charged by a main charger to a first potential and an image region in which a latent electrostatic image is formed by a laser scanning unit to have a second potential, wherein the second potential is lower than the first potential; a development unit for developing the latent electrostatic image using an ink in which toner particles of a predetermined color are dispersed in a liquid carrier; a drying unit for drying a developed toner image; and a transfer unit for transferring a dried image to a print paper, wherein the development unit comprises: a developer roller rotatably installed with a predetermined separation gap from the photoreceptor web, for forming the toner image by attaching the toner particles of the ink to the image region; a toner removal roller rotatably installed with a predetermined separation gap from the photoreceptor web, for removing toner particles remaining in a liquid carrier film adhering to the non-image region; and a squeeze roller rotatably installed in contact with the photoreceptor web, for squeezing the liquid carrier out of the toner image by compressing the toner image.
In one embodiment, the surface of the developer roller is charged to a third potential whose level is between the first and second potentials. In this case, preferably, the third potential is at least 100 volts lower than the first potential.
In another embodiment, the surface of the toner removal roller is charged to a fourth potential whose level is between the potential of the non-image region passed through the developer roller and the potential of the image region passed through the developer roller. Preferably, the fourth potential is at least 50 volts lower than the potential of the non-image region passed through the developer roller. Preferably, the toner removal roller rotates in a direction opposite to a circulation direction of the photoreceptor web.
In still another embodiment, the surface of the squeeze roller is charged to a fifth potential whose level is higher than the first potential so as to recharge the surface of the photoreceptor web to a predetermined potential. Preferably, the squeeze roller is formed of a resistive material having a resistance of 105-109 xcexa9.
Further, a method utilizing the above described apparatus is employed to overcome the problems evident in the prior art.
Thus, according to the present invention, contamination of the development unit and the inks is prevented and image quality is improved.